Since the discovery of photoluminescent porous silicon, see the article by L. T. Canham, "Silicon Quantum Wire Array Fabrication By Electrochemical And Chemical Dissolution Of Wafers", Applied Physics Letters, Vol. 57, No. 10, September 1990, pp. 1096-1098, porous silicon has emerged as a potential photonic source compatible with silicon microelectronics. The light emission mechanism of porous silicon, however, is still not fully understood. A predominant theory is related to the confinement of electrons and holes in a silicon wire or particle with dimensions on the order of tens of nanometers.
In such cases, it is hypothesized that the electrons and holes may recombine and efficiently emit light if there are few non-radiative mechanisms competing for the charge carriers. This theory, called quantum confinement, has been observed in other materials systems and has been theoretically modeled in silicon, see the article: T. Ohno, K. Shiraishi and T. Ogawa, "Intrinsic Origin Of Visible Light Emission From Silicon Quantum Wires: Electronic Structure And Geometrically Restricted Exciton", Physical Review Letters, Vol. 69, No. 16, October 1992, pp. 2400-2403 and the article: S. Sawada, N. Hamada and N. Ookubo, "Mechanisms Of Visible Photoluminescence In Porous Silicon", Physical Review B, Vol. 49, No. 8, 1994, pp. 5236-5245.
These silicon nanoparticles or nanowires are predicted to have a direct band gap, thus allowing efficient electron-hole recombination without employing less efficient phonon-assisted transitions. Other theories note that silicon compounds such as amorphous silicon, silicon oxides, and siloxene derivatives also luminesce in the visible region of the spectrum and propose those models as a source of the luminescence, see the articles: J. M. Perez, J. Villalobos, P. McNeill, J. Prasad, R. Cheek, J. Kelber, J. P. Estrera, P. D. Stevens and R. Glosser, "Direct Evidence For The Amorphous Silicon Phase In Visible Photoluminescent Porous Silicon", Applied Physics Letters, Vol. 61, No. 5, August 1992, pp. 563-565; P. D. Milewski, D. J. Lichtenwalner, P. Mehta, A. Kingon, D. Zhang and R. M. Kolbas, "Light Emission From Crystalline Silicon And Amorphous Silicon Oxide (SiO.sub.x) Nanoparticles", Journal of Electronic Materials, Vol. 23, No. 1, 1994, pp. 57-62; and M. S. Brandt, H. D. Fuchs, M. Stutzmann, J. Weber and M. Cardona, "The Origin Of Visible Luminescence From `Porous Silicon`: A New Interpretation", Solid State Communications, Vol. 81, No. 4, 1992, pp. 307-312.
Irrespective of the complete understanding of the physical mechanism involved, porous silicon properties may be utilized for practical devices.
Typically, the method of fabricating porous silicon in bulk silicon wafers uses an anodic oxidation process with a backside contact to the silicon anode and a platinum (Pt) counter-electrode in a hydrofluoric acid (HF) and ethanol solution, see the Canham article cited above as well as the article by A. Bsiesy, J. C. Vial, F. Gaspard, R. Herino, M. Ligeon, F. Muller, R. Romestain, A. Wasiela, A. Halimaoui and G. Bomchil, "Photoluminescence Of High Porosity And Of Electrochemically Oxidized Porous Silicon Layers", Surface Science, Vol. 254, 1991, pp. 195-200.
Using low current densities, silicon wafers are made porous by the electrochemical dissolution of silicon. Thin films of porous silicon on transparent insulating substrates (sapphire, quartz and the like) have been fabricated which allow analysis of the optical properties from either side of the porous layer and allow the design of novel photonic devices as described in U.S. patent application Ser. No. 08/118,900, filed Sep. 9, 1993 incorporated by reference herein.
Due to the insulating nature of the transparent substrates, the formation of the porous silicon typically use a stain etch composed of hydrofluoric acid (HF), nitric acid (HNO.sub.3) and distilled water in a ratio of 1:5:10 (see R. W. Fathauer, T. George, A. Ksendzov and R. P. Vasquez, "Visible Luminescence From Silicon Wafers Subjected To Stain Etches", Applied Physics Letters, Vol. 60, No. 8, 1992, pp. 995-997; and J. Sarathy, S. Shih, K. Jung, C. Tsai, K. H. Li, D. L. Kwong, J. C. Campbell, S. L. Yau and A. J. Bard, "Demonstration Of Photoluminescence In Nonanodized Silicon", Applied Physics Letters, Vol. 60, No. 13, 1992, pp. 1532-1534) to form the porous layers without need for electrical contact to the backside of the wafer.
Alternatively, a technique described in incorporated U.S. patent application Ser. No. 08/118,900 uses optical excitation rather than electrical contact to form porous silicon. Further description may be found in the articles: W. B. Dubbelday, Szaflarski, D. M., Shimabukuro, R. L., and Russell, S. D., "Study of Photoluminescent Thin Film Porous Silicon On Sapphire", Materials Research Society Symposium Proceedings, Vol. 283, 1993, pp. 161-166 and W. B. Dubbelday, Szaflarski, D. M., Shimabukuro, R. L., Russell, S. D. and Sailor, M. J., "Photoluminescent Thin-film Porous Silicon On Sapphire", Applied Physics Letters, Vol. 62, No. 14, April 1993, pp.1694-1696.
Lauerhaas et al reported in the article identified as: J. M. Lauerhaas, G. M. Credo, J. L. Heinrich and M. J. Sailor "Reversible Luminescence Quenching of Porous Si By Solvents", Journal of the American Chemical Society, Vol. 114, 1992, pp. 1911-1912, that a reversible quenching of photoluminescence is obtained from porous silicon layers fabricated in bulk silicon due to surface adsorbates. In FIG. 1, a cross-section of a prior art porous silicon layer PS formed in a bulk silicon substrate BSS is shown.
The degree of quenching reported by Lauerhaas et al is considered to nominally scale with the solvent dipole moment. Two notable exceptions to this empirical relationship reported by these authors is the lack of quenching observed by exposure to water vapor, and a large quenching of the photoluminescence in the presence of non-polar benzene.
This effect as reported by Lauerhaas et al does not readily lend itself to some device applications due to the need to photoexcite, and detect emitted light from, the porous silicon side of the wafer.
An improved design exploiting the photoluminescence quenching effect of porous silicon will enhance its use in the commercial market.